The present invention relates to a process for producing optical glass by heating and sintering a porous gel prepared by hydrolyzing a metal alkoxide. Particularly, it relates to a process for producing glass for optical parts required to have a low residual water content, for example, optical guides, lenses, prisms, and the like.
There has recently been known a method for producing a non-porous block body (e.g., silica glass body) by sintering a porous gel obtained by hydrolyzing a metal alkoxide (e.g., Si(OCH.sub.3).sub.4) (hereinafter referred to as "a sol-gel method").
As references presenting such a process for producing glass, there may be exemplified M. Yamane et al.; Journal of Materials Science, vol. 14 (1979), pages 607-611. There have been invented, by inventors including the present inventors as main ones, processes for producing optical glass by sintering a high silica gel obtained by hydrolyzing a metal alkoxide (wherein the metal also include Si and Ge). Details of these inventions are disclosed in (a) the specification of a patent application already filed in Japan as Japanese Patent Application No. 3957/79 in which the main inventors are the present inventors, and also filed in U.S.A., Great Britain, West Germany, France, Netherlands and Canada claiming the priority of said Japanese Patent Application, (b) the specifications of patent applications already filed in Japan as Japanese Patent Application Nos. 150082/79, 169335/79, 95755/80 and 95758/80 in which the inventors are the same as the present inventors, and also filed in U.S.A. and Europe (European Patent Application in which Great Britain, West Germany, France and Netherlands are designated) claiming the priorities of these Japanese Patent Applications, (c) the specification of a patent application already filed in Japan as Japanese Patent Application No. 4600/80 in which the inventors are the same as the present inventors, and also filed in U.S.A. and Europe (European Patent Application in which Great Britain, West Germany, France and Netherlands are designated) claiming the priority of said Japanese Patent Application, and (d) the specifications of patent applications already filed in Japan as Japanese patent Application Nos. 55485/80 and 95757/80 in which the inventors are the same as the present inventors, and also filed in U.S.A., Great Britain and West Germany claiming the priorities of these Japanese Patent Applications.
The outline of the aforesaid sol-gel method comprises using, for example, Si(OCH.sub.3).sub.4 as a starting material, mixing and stirring it with CH.sub.3 OH and H.sub.2 O, transferring the resulting mixture into a desired vessel, making the vessel almost air-tight, allowing the vessel to stand to subject the mixture to gelation, gradually lowering the degree of air-tightness of the vessel to evaporate residues (water, alcohol, etc.,) with drying, and then sintering the gel at a temperature of 1,100.degree. C. or higher to make it into glass.
The silica gel obtained by the aforesaid sol-gel method is a porous block having a specific surface area of about 800 m.sup.2 /g and has a pore size of 20 to 100 .ANG., though that depends on the production conditions. When the gel is assumed to be an aggregate of fine particles, it corresponds to an aggregate of fine particles having a size of about 50 .ANG.. When there is heated and sintered as usual such a porous block prepared by the hydrolysis, said block having a large specific surface area, namely, many adsorptive points on surface, the gel sometimes rises in bubbles, and, in an extreme case, it bursts. Main adsorbates causing the above-mentioned rising-in-bubbles include, for example, water and the alcohol which are starting materials for the hydrolysis solution and the chemically adsorbed OCH.sub.3 group, OH group, and the like. Therefore, in sintering the porous gel, the removal of these residues is indispensable.
Concretely, the adsorbed alcohol and water are removed by heating the porous gel to about 300.degree. C., after which the OCH.sub.3 group is subjected to oxidation treatment (called "OCH.sub.3 removal treatment" or "carbon removal treatment") at about 400.degree. C. As to treatment of removing the OH group (called "OH removal treatment" or "hydroxyl group removal treatment"), it is well known to those skilled in the art that it can be achieved to a certain extent by heating the porous gel at 400.degree. C. or higher in an atmosphere of thionyl chloride or chlorine. Since the OH removal treatment is attributable to the substitution reaction of Cl for OH, a rapid reaction is achieved at 700.degree. C. or higher.
When the porous gel subjected to the above-mentioned heating, oxidation and OH removal treatments is heated and sintered, it sometimes rises in bubbles. There are thought to be two causes for this. One of them is that a large amount of Cl substituted for OH in the OH removal treatment is incorporated into the glass by sintering. The other is that the atmosphere gas for sintering is shut up in closed pores. The latter cause is also dependent on the pore distribution in the gel and the like.
When glass rises in bubbles owing to heating, the glass is inevitably extremely limited in use and, in general, becomes unsuitable for optical uses, and it becomes impossible to produce an optical guide by using the glass as a base material.
The following references are cited to show the state of the art; (i) Japanese Patent Appln Kokai (Laid-Open) Nos. 100231/80, 92135/81 and 104732/81, and (ii) M. Yamane et al., Journal of Materials Science, vol. 14 (1979), pages 607-611.